Electron tube of transmission line type



Nov. 3, 1964 J. GILLIES ETAL 3,155,867

ELECTRON TUBE OF TRANSMISSION LINE TYPE Filed June 18, 1962 I 2 Sheets-Sheet 1 Fig.1

5 w v6 NTDR 5 1/2/5455 @144 I65 qua/v 1 6 23151! IT TOP N 5Y5 1954 J. GILLIES ETAL ELECTRON TUBE 0F TRANSMISSION LINE TYPE 2 Sheets-Sheet 2 Filed June 18, 1962 m s 3 m mmmzl w 3. g Wm P/WH F a M J Mat H JH M B United States Patent 3,155,867 ELECTRGN TUBE 0F TRANMISSION LlNE TYPE James Gillies, Stevenage, and Alan Reddish, Pinner, England, assignors to The M-0 Valve Company Limited, London, England, a British company Filed June 18, W62, Ser. No. 203,317 Claims priority, application Great Britain, June 20, 1961, 22,347/61 11 Claims. (Cl. Sis-3.5)

This invention relates to electron tubes.

The invention is concerned in particular with electron tubes of the kind in which in operation a stream of electrons is arranged to travel generally along the length of an interaction space under the influence of electrostatic and magnetic fields which are directed perpendicular to each other and to the length of the interaction space, the electrons being arranged, while in the interaction space, to interact with the electric field of an electromagnetic wave propagated along a transmission line which extends in a direction parallel to the length of the interaction space, the electric field of the wave having an appreciable component which is directed parallel to the electrostatic field in the interaction space and which interacts with the electrons by virtue of their periodic cyclotron motions.

In such an electron tube, in contrast with conventional types of travelling wave tubes, the mode of interaction between the electrons and the electromagnetic wave is such that it is not essential for the transmission line to be of a slow wave type, and in particular it is not essential for the transmission line to incorporate any periodic structure. This is particularly advantageous in tubes intended to operate at very high frequencies, since it makes it possible to avoid the difficulties involved in making periodic structures having elements of very small dimensions.

In an electron tube of the kind specified the interaction may be of either a forward wave type or a backward wave type, that is the electromagnetic wave may travel along the transmission line either in the same sense as, or in the opposite sense to, the general direction of flow of the electrons along the interaction space. In either case the interaction will occur only for a narrow range of signal frequencies, the angular frequency (.0 at which maximum interaction occurs being given by the equation:

where w is the cyclotron frequency and R is the ratio of the general drift velocity V of the electrons along the interaction space to the velocity of the propagation of the wave along the transmission line (R will be positive or negative according to whether the interaction is of a forward wave type or a backward wave type). If, as will usually be the case, the transmission line is not of a slow wave type, the value of R will normally be small compared with unity and the interaction frequency will be close to the cyclotron frequency. An electron tube of the kind specified is tunable over a wide range of frequencies by variation of the magnetic field strength, and is also tunable to some extent by variation of the electrostatic field strength, since the value of the cyclotron frequency w is equal to eB/ m and the value of the drift velocity V is equal to E/B, where e and m are the charge and mass of an electron, B is the magnetic flux density, and E is the strength of the electrostatic field.

Electron tubes of the kind specified may be used either as amplifiers or as self-oscillators, but it would appear that the application of most technical interest is their use as self-oscillators when operating in a backward wave mode.

ice

One difiiculty which has been experienced in designing electron tubes of the kind specified is the provision of a suitable means for injecting the electrons into the interaction space in such a manner as to obtain the desired mode of interaction between the electrons and the electromagnetic wave, and it is an object of the present invention to provide a solution to this problem.

According to the invention, an electron tube of the kind specified has an electron gun disposed adjacent one end of the interaction space and including an electron emissive cathode and a further electrode, the cathode being disposed outside the interaction space beyond that boundary of the interaction space at which the least positive electrostatic potential is established in operation, the electron emissive surface of the cathode and a cooperating surface of the further electrode defining between them a gun space which extends transversely to the length of, and communicates with, the interaction space and in which in operation the magnetic field is established in the same direction as in the interaction space, said surfaces being so disposed and shaped that when the further electrode is maintained at a positive potential With respect to the cathode there is established in the gun space an electrostatic field which has an appreciable component directed perpendicular to the magnetic field in such a sense that electrons emitted from the cathode are constrained to travel through the gun space towards the interaction space, the arrangement being such that in operation, with the cathode maintained approximately at the potential of said boundary and the further electrode maintained at a suitable positive potential with respect to the cathode, electrons are injected into the interaction space with low initial velocities at said boundary.

One arrangement in accordance with the present invention will now be described by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 is a central sectional view of an e ectrou tube designed to generate oscillations at frequencies in the range l2,000-18,000 mc./s.;

FIGURE 2 is a fragmentary side view of the tube, partly cut away to show internal details, on a larger scale than FIGURE 1; and

FIGURE 3 is a sectional view of the tube mounted between the poles of an electromagneh the section being on the line III-III shown in FIGURE 1 and the scale being larger than that of FIGURE 1 but smaller than that of FIGURE 2.

Referring to the drawings, the tube has an evacuated envelope the major part of which is constituted by an elongated copper casing 1 of rectangular cross-section. The end portions of the casing 1 are formed as hollow waveguides 2 and 3 respectively provided at their outer ends with coupling flanges 4 and 5, continuity of the envelope being ensured by means of ceramic windows 6 and 7 respectively sealed across the interiors of the wave guides 2 and 3; the central portionof the casing 1 is of enlarged cross-section and is partially formed by a separate cover 3 which is sealed to the remainder of the casing 1.

The waveguides 2 and 3 have internal cross-sectional dimensions of 1.5 x 0.75 cm., and respectively have formed in them central longitudinally extending ridges 9 and 10 which project from those broad faces of the waveguides 2 and 3 which are nearer the cover 8; each of the ridges 9 and 10 has a uniform width of 8 mm., has at the inner end of the relevant waveguide 2 or 3 a uniform height of 7 mm., and is tapered away towards the outer end of the relevant waveguide 2 or 3.

Two stubs l1 and 12, of rectangular cross-section 8 x 5 mm., respectively project short distances into the central portion of the casing 1 from the ends of the ridges 9 and 1%, each stub 11 or l2'having three of its'faces fiush' with the three lateral faces of the corresponding ridge 9 or 10. Between and accurately aligned with the stubs 11 and 12 are disposed two electrodes 13 and 14 in the form of copper bars having the same cross-section as that of the stubs 11 and 12, the electrodes 13 and 14 being arranged end to end and having lengths such that there are gaps 0.25 mm. wide between the stub 12 and the electrode 13, between the electrodes 13 and 14, and between the stub 11 and the electrode 14. The electrode 13 (which will subsequently be referred to as the anode) is considerably longer than the electrode 14 (which will subsequently be referred to as the collector), and may for example suitably have a length of about 10 cm. It will be appreciated from the foregoing that the inside face of that side wall 15 of the casing 1 which lies opposite the cover 8 is uniformly spaced 0.5 mm. from the adjacent faces of the stubs 11 and 12 and the electrodes 13 and 14.

The electrodes 13 and 14 are respectively secured to two copper tubes 16 and 17 each of which is bent into the shape of a U, the central portions of the tubes 16 and 17 lying partially in longitudinal grooves formed in the electrodes 13 and 14. Each of the end portions of the tubes 16 and 17 passes through a hole such as 18 in the cover 8 and is sealed by means of a flange such as 19 to one end of a ceramic tube such as 20, the other end of which is sealed to the outside of the cover 8 around the relevant hole 18. The tubes 16 and 17 serve as supports and electrical leads for the electrodes 13 and 14, and in operation are arranged to have cooling fluid passed through them so as to cool the electrodes 13 and 14.

The wall 15, which has a thickness of 5 mm., has formed in it a hole 21 of rectangular cross-section, the hole 21 being disposed near that end of the anode 13 which is adjacent the stub 12. The hole 21 is disposed centrally with respect to the width of the wall 15, three of its sides being parallel to the thickness of the wall 15 but the side furthest from the stub 12 being tapered so that the cross-section of the hole increases from the inside to the outside of the wall 15; at the inside face of the wall 15 the hole 21 has a cross-section 8.5 x 5.5 mm., the larger dimension being parallel to the width of the wall 15, and that side of the hole 21 which is nearest the stub 12 lies in a plane spaced 5 mm. from the plane of that end of the anode 13 which is adjacent the stub 12.

The tapering of one side of the hole 21 effectively provides a wedge shaped recess in the wall 15, within which is mounted a thermionic cathode 22 which is in the form of a tubular body, 8 mm. long, of sintered tungsten impregnated with alkaline earth metal compounds; within the cathode 22 is disposed an electrically insulated resistance heater which, for the sake of clarity, is not shown in the drawings. The cathode 22 is disposed with its length parallel to, and arranged centrally with respect to, the width of the wall 15, and has a uniform trapezoidal external cross-section such that one lateral face, which constitutes the electron emissive surface, of the cathode 22 lies in that plane parallel to the thickness of wall 15 which passes through the intersection between the tapered side of the hole 21 and the inside face of the wall 15, a second lateral face of the cathode 22 lies in the plane of the outside face of the wall 15, and a third lateral face of the cathode 22 lies in a plane parallel to, and spaced 0.25 mm. from, the tapered side of the hole 21, this third face having a width of 5 mm.

Within the hole 21 there is also mounted a further electrode 23, which will subsequently be referred to as the plate, in the form of a copper bar 8 mm. long and having a cross-section 5 mm. square. The plate 23 is disposed centrally with respect to the periphery of the hole 21 at the inside of the wall 15, and with two of its lateral faces lying respectively in the planes of the inside and outside faces of the wall 15. It will be appreciated that one lateral face of the plate 23 is disposed parallel to, and spaced 0.25 mm. from, the emissive surface of the cathode 22, and that the lengths of the cathode 22 and 4 the plate 23 lie exactly opposite the width of the anode 13.

The cathode 22 and the plate 23 are supported by means of a support system 24 which is secured to the outside of the casing 1 and is such that the cathode 22 and the plate 23 are electrically insulated from each other and from the casing 1. Electrical leads such as 25 for the plate 23, the cathode 22 and the cathode heater are connected to metal pins such as 26 which are sealed through a glass bulb 27 which is sealed to one end of a metal tube- 28, the other end of which is sealed in an aperture in a metal cap 29 which is in turn sealed to the outside of th casing 1 around the support system 24.

That portion of the wall 15 which extends between the hole 21 and the region opposite the gap between the anode 13 and the collector 14 effectively constitutes yet another electrode, which will subsequently be referred to as the sole. The space between the sole and the anode 13 constitutes an interaction space through which electrons flow in operation of the tube, an electrostatic field directed substantially perpendicular to the opposed faces of the sole and the anode 13 being established in this space by maintaining the anode 13 positive with re spect to the sole (that is positive with respect to the casing 1) in operation.

The cathode 22 and the plate 23 together form an electron gun of a kind similar to one commonly used in conventional M-type travelling wave tubes. The gun space between the emissive surface of the cathode 22 and the opposed face of the plate 23 extends perpendicularly to the length of, and communicates with, the interaction space, and in operation of the tube an electrostatic field directed substantially perpendicular to the emissive surface of the cathode 22 is established in the gun space by maintaining the plate 23 positive with respect to the cathode 22.

The stubs 11 and 12 and the electrodes 13 and 14 effectively constitute one conductor of a two-conductor transmission line, the other conductor of which is effectively constituted principally by the central portion of the casing 1 and partly by the plate 23. The ends of this transmission line are effectively connected respectively to the waveguides 2 and 3, and the line is capable of propagating an electromagnetic wave in a mode such that in the interaction space the electric field of the wave is directed substantially perpendicular to the opposed faces of the sole and the anode 13. It may be desirable to dispose a suitable lossy material (not shown) in the space between the electrodes 13 and 14 and the cover 8 in order to inhibit the propagation of waves in unwanted modes. The various gaps between the elements 11, 12, 13 and 14, and the hole 21, have little effect on the radio frequency properties of the transmission line, and their effect can be minimised by choosing various dimensions of the structure to be equal to one quarter of the wavelength corresponding to a frequency at the centre of the intended operating range; all the dimensions specifically indicated above as having a value of 5 mm. are chosen with this point in view.

The electron tube may be used as a self-oscillator operating in a backward wave mode, in the following manner. As shown in FIGURE 3 the tube is disposed between the poles 30 and 31 of an electromagnet capable of establishing a variable magnetic field having a flux density of several thousand gauss, the tube being situated so that the magnetic field is established in the interaction and gun spaces and is directed parallel to the opposed faces of the sole and the anode 13 and to the emissive surface of the cathode 22; the sense of the magnetic field is made such that the force exerted by the magnetic field on a moving electron is such as to deflect the electron to the right of its original path when viewed as in FIGURE 1. The external end of that waveguide 2 which is connected to the end of the transmission line further from the electron gun is PTO-s amass? vided with a reflectionless termination (not shown), the other waveguides 3 being used as an output connector. The anode 13 is maintained at a potential several thousand volts positive with respect to the sole, the collector 14 is maintained positive with respect to the sole at a potential about one quarter of that of the anode 13, the cathode 22 is maintained at a potential within a few volts of that of the sole, and the plate 23 is maintained at a potential about 350 volts positive with respect to that of the cathode 22; the cathode 22 is heated to a temperature such that the electron emission takes place under space charge limited conditions, the total emitted current being about 200 milliamperes.

The electrons emitted from the cathode 22 are formed, under the influence of the crossed electrostatic and magnetic fields established in the gun space, into a beam wh-ch flows along the gun space towards the interaction space. The electron flow in this beam is of a substantially laminar nature, and the velocities of the electrons in the beam are relatively low, the maximum electron velocity corresponding to an energy of about fifteen electron volts. The electrons are thus injected into the interaction space at one end through the gap between the plate 23 and the edge formed by the intersection of the tapered side of the hole 21 and the inside face of the wall 15, the electrons having low velocities as they pass through the plane of the inside face of the wall 15. This condition for injection of the electrons into the interaction space is desirable in order to achieve a high eiiiciency in the operation of the tube.

In the interaction space the electrons travel generally along the length of the space away from the end adjacent the electron gun under the influence of the crossed electrostatic and magnetic fields established in the interaction space, the electrostatic field of course having a much higher strength than that established in the gun space. The individual electrons follow substantially cycloidal orbits in planes perpendicular to the direction of the magnetic field, the space charge density in the interaction space being sufiiciently low to ensure that these orbits are not smoothed out to any great extent. The motion of any individual electron may be regarded as compounded of a uniform linear motion at the drift velocity V parallel to the length of the interaction space and a circular motion about the direction of the magnetic field at the cyclotron frequency w initially the radius of the cyclotron motion is somewhat less than 0.25 mm. After their passage through the interaction space the electrons flow to the collector 14.

By virtue of the cyclotron motions of the electrons as they travel along the interaction space, there are induced on the conductors of the transmission line alternating potentials such as to set up an electromagnetic wave whose electric field is directed in the interaction space substantially parallel to the electrostatic field in this space, and which travels along the transmission line at the velocity of light in a sense opposite to that of the general direction of the flow of the electrons along the interaction space. As indicated above, the frequency of this wave will be somewhat less than the cyclotron frequency; for example the frequency of the wave will be 15,000 mc./ s. if the magnetic flux density has a value of about 5,600 gauss and the voltage between the anode 13 and the sole is 4,000 volts. Thus an output signal of variable frequency may be derived from the waveguide 3.

The etficiency of the process by which the wave is generated is dependent on the following mechanism. It will be appreciated that electrons entering the interaction space at different instants will experience different phases of the electric field of the wave, and that consequently some of the electrons will initially have cyclotron motions of phases such that they gain energy from the field and some will initially have cyclotron motions of phases such that they lose energy to the field. For those electrons which initially gain energy from the field,

the radii of the cyclotron motions will be increased, and consequently on completing the first loop of their cycloidal orbits most of these electrons will strike the sole and be removed from the interaction space; little energy is thereby dissipated since the electrons strike the sole with relatively low velocities. On the other hand for those electrons which initially lose energy to the field, the radii of the cyclotron motions will be decreased, and consequently these electrons will continue to travel along the interaction space after completing the first loop of their cycloidal orbits, the electrons then continuing to lose energy to the field so that the radii of their cyclotron motions become smaller and smaller. Thus the electrons which participate to the greatest extent in the interaction process will be very largely confined to those which enter the interaction space at instants which are favourable from the point of view of maintaining the Wave. The sorting mechanism described above is particularly effective where the backward wave type of interaction is involved, since in this case the electric field of the Wave is strongest at that end of the interaction space at which the electrons are injected.

We claim:

1. An electron tube comprising:

an evacuated envelope;

a transmission line effectively in the form of a spaced apart pair of elongated conductors whose lengths are generally parallel to each other, a portion of the space between the conductors constituting an elongated interaction space whose length is generally parallel to the lengths of the conductors, the interaction space being situated within the envelope and having first and second lateral boundaries respectively defined by portions of the two conductors, whereby an electromagnetic wave may be propagated along the transmission line in a mode such that in the interaction space the electric field of the Wave has an appreciable component in the direction of the spacing between said lateral boundaries;

an anode electrode and a sole electrode respectively constituted by parts of the two conductors such that the anode electrode and the sole electrode have surfaces which respectively coincide with said first and second lateral boundaries, the relevant surface of the sole electrode having a first edge which corresponds to a first end of the interaction space;

means whereby the anode electrode may be maintained at a positive potential with respect to the sole electrode so as to establish within the interaction space an electrostatic field in the direction of the spacing between said lateral boundaries;

an electron gun disposed adjacent said first end of the interact-ion space so as to lie to that side of the interaction space corresponding to said second lateral boundary, the electron gun including a cathode having an electron emissive surface and a plate electrode having a surface which faces the electron emissive surface so that there is formed between these two surfaces a gun space which is situated within the envelope, said two surfaces extending transversely to the direction of the length of the interaction space at said first end of the interaction space and being disposed so that motion in that direction towards the emissive surface from the facing surface of the plate electrode corresponds to motion along the interaction space from said first end to the other end, and said two surfaces each having an edge disposed adjacent said first edge so that the gun space is effectively contiguous with the interaction space in the vicinity of said first edge;

means whereby the cathode may be maintained at least approximately at the same potential as the sole electrode;

and means whereby the plate electrode may be maintamed at a positive potential with respect to the cathode so as to establish within the gun space an electrostatic field extending between the emissive surface of the cathode and the facing surface of the plate electrode;

the tube being adapted to operate with a magnetic field established in the interaction and gun spaces in a direction perpendicular to the length of the interaction space and to the direction of the spacing between said lateral boundaries and with a sense such that, under the influence of the magnetic field and said electrostatic fields, electrons emitted from the cathode are caused to travel through the gun space towards the interaction space and, upon being injected from the gun space into the interaction space in the vicinity of said first edge, are caused to travel generally along the length of the interaction space from said first end to the other end.

2. An electron tube according to claim 1, in which said emissive surface lies in a plane disposed perpendicular to the direction of the length of the interaction space at said first end of the interaction space.

3. An electron tube according to claim 1, in which said emissive surface lies in a plane which passes through said first edge.

4. An electron tube according to claim 1, in which said emissive surface is parallel to the facing surface of the plate electrode.

5. An electron tube according to claim 1, in which the lengths of the conductors of the transmission line are substantially rectilinear.

6. An electron tube according to claim 5, in which the two conductors of the transmission line respectively have planar faces which are disposed parallel to and facing each other, parts of the planar faces respectively 8 constituting those surfaces of the anode electrode and the sole electrode which respectively coincide with said first and second lateral boundaries of the interaction space.

7. An electron tube according in claim 6, in which that one of the conductors of which a portion constitutes the sole electrode has formed in it an aperture which extends to said planar face of that conductor, said first edge constituting part of the boundary of the aperture at said planar face, and the cathode and plate electrode are disposed in said aperture with the plate electrode having a surface lying in the plane of said planar face.

8. An electron tube according to claim 5, in which one of the conductors of the transmission line surrounds the other.

9. An electron tube according to claim 8, in which said one of the conductors is that one of which a portion constitutes the sole electrode.

10. An electron tube according to claim 8, in which the envelope is partially constituted by part of said one of the conductors.

11. An electron tube according to claim 1, in which the transmission line has an end portion extending longitudinally away from the interaction space beyond said first end of the interaction space and beyond the electron gun, and in which the tube is provided with a radio frequency connector coupled to said end portion of the transmission line.

References Cited in the file of this patent UNITED STATES PATENTS 2,804,569 Huber Aug. 27, 1957 2,807,739 Berterottiere et al. Sept. 24, 1957 

1. AN ELECTRON TUBE COMPRISING: AN EVACUATED ENVELOPE; A TRANSMISSION LINE EFFECTIVELY IN THE FORM OF A SPACED APART PAIR OF ELONGATED CONDUCTORS WHOSE LENGTHS ARE GENERALLY PARALLEL TO EACH OTHER, A PORTION OF THE SPACE BETWEEN THE CONDUCTORS CONSTITUTING AN ELONGATED INTERACTION SPACE WHOSE LENGTH IS GENERALLY PARALLEL TO THE LENGTHS OF THE CONDUCTORS, THE INTERACTION SPACE BEING SITUATED WITHIN THE ENVELOPE AND HAVING FIRST AND SECOND LATERAL BOUNDRIES RESPECTIVELY DEFINED BY PORTIONS OF THE TWO CONDUCTORS, WHEREBY AN ELECTROMAGNETIC WAVE MAY BE PROPAGATED ALONG THE TRANSMISSION LINE IN A MODE SUCH THAT IN THE INTERACTION SPACE THE ELECTRIC FIELD OF THE WAVE HAS AN APPRECIABLE COMPONENT IN THE DIRECTION OF THE SPACING BETWEEN SAID LATERAL BOUNDRIES; AN ANODE ELECTRODE AND A SOLE ELECTRODE RESPECTIVELY CONSTITUTED BY PARTS OF THE TWO CONDUCTORS SUCH THAT THE ANODE ELECTRODE AND THE SOLE ELECTRODE HAVE SURFACES WHICH RESPECTIVELY COINCIDE WITH SAID FIRST AND SECOND LATERAL BOUNDARIES, THE RELEVANT SURFACE OF THE SOLE ELECTRODE HAVING A FIRST EDGE WHICH CORRESPONDS TO A FIRST END OF THE INTERACTION SPACE; MEANS WHEREBY THE ANODE ELECTRODE MAY BE MAINTAINED AT A POSITIVE POTENTIAL WITH RESPECT TO THE SOLE ELECTRODE SO AS TO ESTABLISH WITHIN THE INTERACTION SPACE AN ELECTROSTATIC FIELD IN THE DIRECTION OF THE SPACING BETWEEN SAID LATERAL BOUNDARIES; AN ELECTRON GUN DISPOSED ADJACENT SAID FIRST END OF THE INTERACTION SPACE SO AS TO LIE TO THAT SIDE OF THE INTERACTION SPACE CORRESPONDING TO SAID SECOND LATERAL BOUNDARY, THE ELECTRON GUN INCLUDING A CATHODE HAVING AN ELECTRON EMISSIVE SURFACE AND A PLATE ELECTRODE HAVING A SURFACE WHICH FACES THE ELECTRON EMISSIVE SURFACE SO THAT THERE IS FORMED BETWEEN THESE TWO SURFACES A GUN SPACE WHICH IS SITUATED WITHIN THE ENVELOPE, SAID TWO SURFACES EXTENDING TRANSVERSELY TO THE DIRECTION OF THE LENGTH OF THE INTERACTION SPACE AT SAID FIRST END OF THE INTERACTION SPACE AND BEING DISPOSED SO THAT MOTION IN THAT DIRECTION TOWARDS THE EMISSIVE SURFACE FROM THE FACING SURFACE OF THE PLATE ELECTRODE CORRESPONDS TO MOTION ALONG THE INTERACTION SPACE FROM SAID FIRST END TO THE OTHER END, AND SAID TWO SURFACES EACH HAVING AN EDGE DISPOSED ADJACENT SAID FIRST EDGE SO THAT THE GUN SPACE IS EFFECTIVELY CONTIGUOUS WITH THE INTERACTION SPACE IN THE VICINITY OF SAID FIRST EDGE; MEANS WHEREBY THE CATHODE MAY BE MAINTAINED AT LEAST APPROXIMATELY AT THE SAME POTENTIAL AS THE SOLE ELECTRODE; AND MEANS WHEREBY THE PLATE ELECTRODE MAY BE MAINTAINED AT A POSITIVE POTENTIAL WITH RESPECT TO THE CATHODE SO AS TO ESTABLISH WITHIN THE GUN SPACE AN ELECTROSTATIC FIELD EXTENDING BETWEEN THE EMISSIVE SURFACE OF THE CATHODE AND THE FACING SURFACE OF THE PLATE ELECTRODE; THE TUBE BEING ADAPTED TO OPERATE WITH A MAGNETIC FIELD ESTABLISHED IN THE INTERACTION AND GUN SPACES IN A DIRECTION PERPENDICULAR TO THE LENGTH OF THE INTERACTION SPACE AND TO THE DIRECTION OF THE SPACING BETWEEN SAID LATERAL BOUNDARIES AND WITH A SENSE SUCH THAT, UNDER THE INFLUENCE OF THE MAGNETIC FIELD AND SAID ELECTROSTATIC FIELDS, ELECTRONS EMITTED FROM THE CATHODE ARE CAUSED TO TRAVEL THROUGH THE GUN SPACE TOWARDS THE INTERACTION SPACE AND, UPON BEING INJECTED FROM THE GUN SPACE INTO THE INTERACTION SPACE IN THE VICINITY OF SAID FIRST EDGE, ARE CAUSED TO TRAVEL GENERALLY ALONG THE LENGTH OF THE INTERACTION SPACE FROM SAID FIRST END TO THE OTHER END. 